Titanium descaling bath and process



2,790,733 7 TITANIUM DESCALING BATH AND rnocnss Henry L. Alexander, Wilmington, Del., and Hugh Farrell, Niagara Falls, N. Y., assignors to E. I. du Pont de Nemours & Company, Wilmington, Del., :1 corporation of Delaware No Drawing. Application April 14, 1955, Serial No. 501,434

8 Claims. (Cl. 134-2) This invention relates to a novel fused salt bath and process for descaling metallic titanium.

Although titanium is a metal fairly resistant to corrosion, it does develop an oxide scale which must be removed at various stagesin its fabrication. Several methods have heretofore been used to accomplish this removal but each suffers from disadvantages. Milling or abrasive operations, sometimes used, are too wasteful of the valuable metal for real commercial success. Acid etches, also employed, in like manner cause great loss of the metal.

Scaled titanium has also been treated in a fused caustic bath containing sodium hydride. This bath alters the scale so that the latter can quickly be removed by acid. In one procedure the titanium is immersed in fused caustic containing around 1.7% sodium hydride for about two minutes at around 750 F. a The scale is converted to a form easily removed by dissolution in mixed nitric and hydrofluoric acids within about fifteen seconds. A While the combined hydride-acid treatment is effective in removing the scale, it is generally impossible to restrict the caustic immersion to only two minutes or the time required tosolubilize the scale. Immersion pro longed beyond this period is very harmful to the base metal since it introduces hydrogen thereinto. Dissolved hydrogen causes embrittlement of the metal and consequent failure under stress. Commercial specifications may require that the titanium contain less than 125-l50 p. p. m. of hydrogen. This quantity of hydrogen is rapidly absorbed from molten caustic alone. I

An over-all object of this invention is, therefore, provision of a novel and useful method for descaling titanium. Another object is provision of a method for descaling titanium which does not waste the metal. An additional object is provision of a method for descaling titanium which does not cause hydrogen embrittlement thereof. A further object'of the invention is provision of a fused salt bath'for descaling titanium which does not cause undue loss or hydrogen embrittlement of the metal.

The above-mentioned and still further objects may be achieved in accordance with this invention by a process utilizing a fused caustic bath containing some sodium hydride and, additionally, titanium dioxide. The quantity of titanium dioxide used is not sharply critical but if specifications as to hydrogen content are to be met, the

additive should comprise about 0.20-0.S%, by weight of the bath. The upper limit, about 0.5% but dependent to some extent on the temperature, is the saturation point of the titanium dioxide. some reduction in hydrogen absorption may be noted when less than 0.2% by weight of titanium dioxide are used but the improvement'is insufficientto meet commercial standards. The molten bath, here modified to descale titanium,'is basically that of U. S. Patent2,377,87 6. This patent shows fused alkali metal, hydroxides containing l20% of thealkaligmetal hydride as. descaling'baths. In ..the present invention: the quantity of alkali metal hydride used above, titanium metal dissolves very slowly.

2,790,738 Patented Apr. 30, 1957 is not critical and may vary up to saturation. The bydride may also be entirely omitted if the hydroxide employed is anhydrous. Since the presence of the hydride insures that the bath is anhydrous, 0.5-2% at least of this compound is preferably included therein. A bath containing hydride as well as titanium dioxide possesses the advantage that it can be used for descaling steels in addition to titanium.

The hydride may be prepared by known methods in the bath itself, i. e., by reacting alkali metal with hydrogen or dissociated ammonia in the molten salt. Alternatively but less preferably, the hydride may be added preformed to the bath. I

The alkali metal used in making up the bath is, most conveniently, sodium. Thus a preferred bath comprises molten caustic containing sodium hydride. Other alkali metals, potassium for example, can be substituted in whole or in part for the sodium. 7

Titanium dioxide may be added to the bath in any convenient form. Thus white titanium dioxide pigment of either the anatase or rutile forms, ilmenite ore, rutile ore, anatase ore and beneficiated titanium ores can alike be used. Alkali metal titanates, as for example sodium titanates in which the NazO/TiOz ratio may vary from 0 to 2 or more, can also be employed. Any of the foregoingmaterials should be added with caution to a bath containing a hydride since violent reaction with the latter may take place. Titanium metal is another source of titanium dioxide and can be dissolved directly in the fused bath. The dissolution of the metal is, however, too slow for practical utility.

Whatever method of supplying the titanium dioxide is chosen, it may be necessary to replenish the compound at intervals. Drag-out on the workpieces treated reduces the concentration below a beneficial level. As noted In addition, articles being descaled contact the bath for a short time only. In consequence, dissolution of the titanium does not compensate for titanium dioxide dragged from the bath.

The operation of the bath is relatively simple. Titanium articles are merely dipped into the molten salts and withdrawn. The two most important variables connected with the immersion are the temperature of the bath and the time of contact between it and the workpiece.

-' The molten baths operate most effectively between about 675 F. and 800 F. The preferred temperature is around 700 F. Lower temperatures down to the melting point of the baths, i. e., about 625 F., can be employed. Baths operated in the lower range are more difiicult to control than baths operated in the preferred range.

The absorption of hydrogen by the titanium is cumu lative and dependent upon the contact time. The latter must, consequently, be kept rather low. An immersion of' about two minutes suffices for most of the scale normally encountered on titanium. An immersion of ten or fifteen minutes or even longer can be used in the baths of this invention without excessively embrittling plete removal of the scale. Any conventional acid rinse can be used. In a preferred rinse the workpiece is dipped at room temperature for two minutes in a solution containing 20-30 volumes of nitric acid of around 70% by Weight in concentration, 24 volumes of hydrofluoric acid of around 50% by weight in concentration and 78-67 volumes of water. Satisfactory baths and pickling methods are discussed in Rem-Cru Titanium Reviewf 2, No. 4 (October 1954). Air-drying can then be conveniently used after the acid treatment.

An advantage of the present descaling baths is that they are not damaged by the presence of the carbonate ion. They can thus be left open to the atmosphere. Carbonate up to saturation may aid in repressing the absorption of hydrogen. In one preferred embodiment of the invention an alkali metal carbonate such as sodium carbonate is actually added in a concentration of about 1-20% by weight to the bath. To prevent excessive absorption of carbon dioxide, conventional covers such as floating pans or finely divided charcoal or graphite may be used as desired. These covers also prevent excess loss of sodium hydride.

There follow some examples which illustrate, but do not limit, the invention in more detail. All percentages in these examples are by weight.

EXAMPLE 1 This example shows hydrogen absorption from a bath prepared in accordance with my invention.

a. One and one-half kilograms of sodium hydroxide was melted in a stainless-steel container and raised to a temperature of 740 F. Sufficient sodium monoxide was added to the melt to remove the last traces of moisture therefrom.

Twenty-six grams of sodium metal was then added to the caustic and hydrogen gas passed through it for one hour. A concentration of 1.7% of sodium hydride re sulted.

To the molten caustic-hydride mixture was finally added 26.5 grams of pigment-grade titanium dioxide, this amount being sufiicient to insure saturation with the latter compound.

b. Into the bath prepared as described above were dipped, for various periods of time, pieces of commercial titanium. At the conclusion of its immersion period each piece was quneched in water. It was then successively treated for 15 seconds at room temperature in a solution containing 10% nitric and 3% hydrofluoric acid, rinsed and air-dried. Table I shows hydrogen absorption and weight losses from these samples.

Table I RESULTS WITH BATH SATURATED WITH TITANIUIH DIOXIDE This example is included as a control.

The runs of Example 1 were substantially repeated except that the bath utilized contained no titanium dioxide. Hydrogen absorption and weight loss are shown in Table II.

4 Table II Period of Hydrogen Weight Lost Immersion Absorbed (mgJomfi) (Minutes) (1). p. in.)

Run

Comparison with Table I shows that titanium dioxide not only reduces hydrogen absorption in hydride baths to safe minima but also greatly reduces the loss of weight of the titanium workpiece treated.

EXAMPLE 3 Example 1 was substantially repeated in a series of runs designed to demonstrate the importance of titanium dioxide concentration. The period of immersion in the bath was twenty minutes in each instance. Results are shown in Table III. In this table runs with the same number but labeled a and b, respectively, were made simultaneously in the same bath.

Table III EFFECT OF VARYING TITANIUM DIOXIDE CONCENTRATION Iemp., Percent Percent Hydrogen Wt. Loss Run F. NaH T10; Absorbed (man/em T l. 0. 00 629 7. 48 750 1. 7 0. 083 205 5. 75 750 2. 0 0.158 221 3. 24 750 2. (l 0. 158 228 3. 24 750 1. 7 0.196 30 1.1 750 1. 7 0.196 26 1.1 700 1. 7 0.30 22 0. 40 700 1. 0 0. 33 18 0.62 750 1.7 (1.4 17 0.71 750 1. 7 0.4 0.45 750 l. O 0. 425 12 0. 66 750 1.0 0.425 66 750 1. 7 0. 48 19 1. 49 750 1.85 0.513 17 1. 00 750 1. 0. 513 18 0. G3

EXAMPLE 4 Example 1 was substantially repeated in a series of experiments designed to show the effect of sodium carbonate. Results are shown in Table IV. The baths used contained 0.6% of sodium hydride and were approximately saturated with titanium dioxide. The bath of the last three runs contained 20% of sodium carbonate.

Table IV EFFECTS OF CARBONAIE Hydrogen Content 1. p. m.) Tempera- Run Time ture (Minutes) F.) Before After Treat- Treat- Increase merit ment Various modifications of our invention will be apparent to those skilled in the chemical arts. Thus chemicals other than sodium carbonate or hydride can be present in the caustic baths as desired. The primary requirement is that these chemicals be compatible with the caustic, titanium dioxide and titanium metal, i. e., that they not react deleteriously with the other bath ingredients or with the workpiece. Consequently we pro pose to be bound solely by the appended claims.

Having described our invention, we claim:

1. A fused bath for descaling a titanium workpiece comprising a hydroxide of an alkali metal containing sufficient dissolved titanium to repress the absorption of hydrogen by said workpiece, the dissolved titanium, measured as titanium dioxide, forming about 0.2-0.5% by weight of said bath.

2. The bath of claim 1 in which the hydroxide is sodium hydroxide.

3. The bath of claim 1 containing additionally sodium hydride.

4. In the process of descaling a titanium workpiece, the steps of (1) contacting said workpiece with a fused bath comprising a hydroxide of an alkali metal containing sufficient dissolved titanium to repress the absorption of hydrogen by said workpiece, the dissolved titanium, measured as titanium dioxide, forming about 02-05% by weight of said bath, and (2) subsequently removing the reduced scale from said workpiece.

5. The process of claim 4 in which the hydroxide is sodium hydroxide.

6. The process of claim 4 in which the bath contains additionally sodium hydride.

7. The process of claim 4 in which the temperature of the fused bath is about 625 -800 F.

8. The process of claim 4 in which the time of contact between the workpiece and the fused bath is about 2-15 minutes. 7

Handbook on Titanium Metal, 7th ed., pp. 88, 89, published by Titanium Metals Corporation, New York (August 1, 1953). 

4. IN THE PROCESS OF DESCATING A TITANIUM WORKPIECE, THE STEPS OF (1) CONTACTING SAID WORKPIECE WITH A FUSED BATH COMPRISING A HYDROXIDE OF AN ALKALI METAL CONTAINING SUFFICIENT DISSOLVED TITANIUM TO REPRESS THE ABSORPTION OF HYDROGEN BY SAID WORKPIECE, THE DISSOLVED TITANIUM, MEASURED AS TITANIUM DIOXIDE, FORMING ABOUT 0.2-0.5% BY WEIGHT OF SAID BATH, AND (2) SUBSEQUENTLY REMOVING THE REDUCED SCALE FROM SAID WORKPIECE. 